1. Field of the Invention
The present invention relates to an interconnect structure and, more particularly, to a copper-topped interconnect structure that has thin and thick copper traces, and a method of forming the copper-topped interconnect structure.
2. Description of the Related Art
A metal interconnect structure is a multi-layered structure that electrically interconnects together the various devices formed on a semiconductor wafer to realize an electrical circuit. In order to lower the resistance of the interconnect structure, the top layer of the interconnect structure is commonly formed from copper.
FIGS. 1A-1E show a series of cross-sectional views that illustrate a prior-art method 100 of forming a copper-topped interconnect structure. As shown in FIG. 1A, method 100 utilizes a conventionally-formed semiconductor wafer 110 that has an interconnect structure which includes a non-conductive region and a number of conductive structures 112, such as aluminum traces, that touch and sit on the non-conductive region.
As further shown in FIG. 1A, method 100 begins by depositing a layer of passivation material 114 on the non-conductive region and the conductive structures 112. Method 100 continues by forming and patterning a mask 116 on passivation layer 114. Following this, the exposed regions of passivation layer 114 are etched to form a number of openings 120. Some of the openings 120, in turn, expose the conductive structures 112. Mask 116 is then removed.
As shown in FIG. 1B, after mask 116 has been removed, a seed layer 122 is formed on the conductive structures 112 and passivation layer 114. Seed layer 122 typically includes a layer of titanium (e.g., 300 Å thick) and an overlying layer of copper (e.g., 3000 Å thick). The titanium layer enhances the adhesion between the underlying aluminum traces 112 and the overlying layer of copper. (Seed layer 122 can also include an overlying layer of titanium, which is stripped before plating. In addition, seed layer 122 can further include a conductive barrier layer that lies between the aluminum traces 112 and the lower titanium layer.) Next, after seed layer 122 has been formed, a plating mold 124 is formed on seed layer 122 to have a number of openings 126 that expose the number of openings 120.
As shown in FIG. 1C, following the formation of plating mold 124, copper is electroplated to form a number of copper traces 130 in plating mold 124. The copper traces 130, which are electrically connected to the conductive structures 112, are separated from each other by a substantially uniform minimum distance MD. In addition, each copper trace 130 has a top surface 130T and a thickness of approximately 5 μm. After the electroplating process has been completed, plating mold 124 and the seed layer 122 that underlies plating mold 124 are then removed.
Next, as shown in FIG. 1D, a layer of non-conductive material (e.g., benzocyclobutene (BCB) or a polymer) 132 is deposited on passivation layer 114 and the copper traces 130. After non-conductive layer 132 has been deposited, a mask 134 is formed on non-conductive layer 132. Following this, the exposed regions of non-conductive layer 132 are etched to form openings 136 that expose the copper traces 130. Mask 134 is then removed.
As shown in FIG. 1E, after the openings 136 in non-conductive layer 132 have been formed, a metal layer 138 is formed on non-conductive layer 132 to fill up the openings 134 and contact the copper traces 130. Metal layer 138 can be implemented with, for example, gold or aluminum with an underlying titanium layer. The titanium layer enhances the adhesion of the aluminum to the copper.
After this, a mask 140 is formed and patterned on metal layer 138. Next, the exposed regions of metal layer 138 are etched to form a number of metal bond pads 142 over selected regions of the top surfaces of the copper traces 130. Mask 140 is then removed. After mask 140 has been removed, solder balls can be attached to the metal bond pads 142 or, alternately, bonding wires can be attached to the metal bond pads 142.
Although method 100 provides an approach to forming a copper-topped interconnect structure, there is a need for additional methods of forming copper-topped interconnect structures.